The speed in which a PIC will operate is a division of 4 of the crystal, and TMR0 will increment each instruction cycle depending on the pre-scaler, and a TMR0 overflow interrupt will occur every time TMR0 overflows from 255 to 0.
So a 10Mhz crystal will have the PIC operating at 2.5Mhz. This means that every second, 2.5 million instructions are executed.
The TMR0 interrupt (with a prescaler of 1:2) will occur every 512 cycles, so 2.5Mhz / 512 = 4882.8125 interrupts a second (or 0.0002048 seconds = 204.8uS)
If you change the Pre-Scaler too 1:4 then an interrupt will occur once every 1024 Cycles, so 2.5Mhz / 1024 = 2441.40625 interrupts a second (or 0.0004096 seconds = 409.6uS)